The present invention relates to a method and a device for cleaning extrusion heads during the extrusion of thermoplastic plastic materials, which are expelled (extruded) from an extrusion nozzle.
When processing thermoplastic plastic materials, such as polyolefins—predominantly with medium- and high-molecular polyethylene types, such as HDPE, for example, during extrusion blow molding, in particular during continuous extrusion with continuous tube discharge, but also with discontinuous tube discharge (storage heads, accumulator-heads), the following disadvantageous effect occurs:
Ejected liquid molten plastic is under high pressure in the extrusion device (e.g., extrusion head). After exiting the extrusion nozzle, the plastic relaxes to atmospheric pressure and thereby swells. During the exiting process, gel-type products (lubricants, additives) are segregated from the hot plastic strand over time, in particular with continuous extrusion, and are deposited as harmful material buildup in the proximate region of the nozzle mouth, with ring-shaped nozzles primarily in the interior on the core, but also on the exterior nozzle mouthpiece. Ring-shaped discharge nozzles therefore have the problem that these deposits are also not visible. The deposits (precipitates) result from low-molecular components of the polymer melt (e.g., short polymer chains, additives, wax-like components), which deposit on the discharge edges of the nozzle channel when the melt exits due to the sudden decrease in pressure and the decrease in the wall shear stress.
The gel-type products are continuously exposed to high temperatures and are transformed into resin and coke within a relatively short time. These progressively growing deposits become brittle and then break off or disintegrate uncontrollably. These crumbs or chunks, which can be several centimeters long, fall into the extruded tube and damage the finished products (for example, when producing a drum, leaky weld seams can be created on the bottom when such porous coke particles enter the region of the weld seam).
For example, the particles may only loosely adhere to the interior wall of the blown finished product (e.g., a tight-head drum) and may break off and fall off when the hollow body is filled with the actual charge, and may thus reach and contaminate the charge.
The deposits (gel-type products, takings) have initially a low-viscosity consistency. The deposits oxidize over time due to the high temperatures of nozzle and core and through exposure to oxygen from the ambient air, and the viscosity increases until first solid regions are formed by coking.
The outside deposits on the discharge nozzle grow uncontrollably. In particular in the blow molding technique, where a tubular preform is expelled from a ring-shaped nozzle, in particular during continuous melt discharge, the inner core region of the discharge nozzle is not visible.
When the deposits have grown too much and become brittle, they are carried along from time to time by the inside of the passing melt tube and adhere on the interior surface of the produced product, for example a plastic tight-head drum. This disadvantageously causes contamination of the charge after the drum has been filled.
During a subsequent detachment from the interior wall of a drum, this may cause feed pumps to become plugged or damaged and may significantly impair the subsequent processing processes of the charge. If such coke particles reside in the region of the weld seam of the blow-molded container, then this can disadvantageously and impermissibly weaken the container (leaks).
Moreover, such deposits may cause longitudinal stripes to form, causing optical and/or technical defects of the extruded and subsequently blow-molded product.
This phenomenon is also referred to as “die build-up”, wherein the deposits of thermally damaged polymers on the nozzle exit cause undesirable surface irregularities. In commercially available plastics, individual additives in general(such as stabilizers or flow promoters) for influencing wall slip effects lead to or cause sticky deposits.
These problems are known, for example, from the technical field of blown film extrusion. The deposits are here stripped with mechanical cleaning devices, such as scrapers, and transported to the waste area, which is during film blowing always produced when the winding spindles are changed.
However, in the cleaning process of extrusion tools practiced to date, the extrusion process must be stopped for performing the partially manual mechanical cleaning of the nozzle core directly below the discharge edge of the melt.
Stopping the extrusion process causes a loss in production due to the idle time; on the other hand, the facility must then be started up again by a knowledgeable process engineer and placed into a stable operating state.